# First order pde cauchy problem by method of characteristics

1. Jul 15, 2008

### pk415

Ok, so I can get through most of this but I can't seem to get the last part... Here is the problem

$$xU_x + (y^2+1)U_y = U-1; U(x,x) = e^x$$

Characteristic equations are:

$$\frac{dx}{x} = \frac{dy}{y^2+1} = \frac{dU}{U-1}$$

Solving the first and third gives:

$$\frac{U-1}{x} = c_1$$

The first and second equation yield:

$$tan^{-1}(y) - lnx = c_2$$

Put the two together in the form

$$c_1 = f(c_2)$$

$$\frac{U-1}{x} = f(tan^{-1}(y) - lnx)$$

Sub in the Cauchy data and you get

$$\frac{e^x-1}{x} = f(tan^{-1}(x) - lnx)$$

Now how do I find what my arbitrary function f is? I have spent hours on this. Is there something that relates inverse tan to natural log? Arrggghhhh!

Thanks for any help.

2. Jul 16, 2008

### Big-T

Last edited by a moderator: Apr 23, 2017